1,720,961 research outputs found
Hybridization of an internal combustion engine with a molten carbonate fuel cell for marine applications
No full textThis study presents a proposed hybrid ship propulsion system combining an internal combustion engine and a molten carbonate fuel cell both powered by liquefied natural gas (LNG). Exhaust from the internal combustion engine is used as a CO2 source for cell operation, reducing CO2 emissions. Use of fuel stored at very low temperature requires heat for evaporation purposes. The fuel is used to condense water vapor from the fuel cell exhaust gases, returning the remainder to the fuel cell with the right amount of water. This solution increases the electricity generation efficiency of the fuel cell. We analyzed two different system configurations that differ in the way the anode off-gas is recirculated. In the first, all the unoxidized fuel is recirculated to the anode inlet; in the second, off-gas is joined with engine flue gas, and residual fuel burned in a combustion chamber before being sent to the cathode of the fuel cell, allowing to maintain an optimal CO2:O2 ratio in the cathode flow of the fuel cell. A detailed numerical model of the system including cell operation was created in Aspen Hysys and optimized to maximize the system efficiency. Results showed that in configuration I the efficiency gain is about 4.9% with respect to the traditional engine. In configuration II the efficiency gain was only about 0.8%. We also analyzed the sensitivity of the systems from the point of view of the limitations occurring here (e.g., steam-to-carbon ratio or operating temperature). Finally, we discussed the size of such a fuel cell in relation to the internal combustion engine, the entire ship, as well as the impact of the increase in efficiency on the range of the vessel
Small-scale desalination plant driven by solar energy for isolated communities
Full text linkIn the last years, an increasing number of countries has been affected by water shortage. Seawater desalination driven by solar energy, which is usually available in arid regions, might be a solution to satisfy the freshwater demand. In this study, the feasibility of a stand-alone multi-effect desalination (MED) plant driven by solar energy for an isolated community was studied. The system was made up of a solar field, a MED unit, and a thermal storage that mitigated solar energy fluctuations. Simulations with different top brine temperature and inclination and number of the solar panels were carried out in Matlab and Aspen Plus on an hourly basis by considering one typical meteorological year for ambient temperature and solar radiation. Two different sources of electrical energy were considered: A photovoltaic (PV) field and a diesel generator. The results were compared from an energetic and economic point of view, by considering the adoption of plastic as a material for MED heat exchangers. The maximum water production was obtained with December as the design month. Polytetrafluoroethylene heat exchangers allowed the cost of water to be reduced up to 9.5% compared to conventional exchangers. The lowest cost of water (7.09 $/m3) was obtained with September as the design month and a tilt angle of 45◦ with the PV field as the electrical power source
Continuous decentralized hydrogen production through alkaline water electrolysis powered by an oxygen-enriched air integrated biomass gasification combined cycle
No full textThis research work presents an innovative approach for continuous decentralized production of renewable hydrogen from woody biomass. Alkaline water electrolysis (AWE) is used to produce high-purity hydrogen, while the oxygen by-product is mixed with ambient air and used to fire a biomass-fueled downdraft gasifier in order to produce an upgraded producer gas with a lower heating value (LHV) between 7-8 MJ/Nm3. This fuel gas is then subjected to a conditioning stage and eventually fed to a combined cycle consisting of a recuperative gas turbine as topping unit and a regenerative subcritical organic Rankine cycle as bottoming unit, which together allow for a combined electric power generation efficiency close to 40%. Most of the net AC power from the integrated gasification combined cycle (IGCC) is rectified to DC power and ultimately used to power an alkaline electrolyzer, with a minor share allocated to all the required utilities and ancillary equipment, including hydrogen compression to 200 bar. The results from simulation of the hybrid IGCC-AWE plant under steady-state operating conditions in Aspen Plus V.11 indicate an optimal efficiency of 17.6% based on the LHV of hydrogen. Thus, if sized for a biomass consumption of 1 t/h, the proposed plant is capable of providing around 26 kg/h of compressed hydrogen at 200 bar
Dynamic model of a power-to-gas system: Role of hydrogen storage and management strategies
No full textTackling climate change requires a drastic decarbonization of the energy system through the introduction of renewable energy technologies. However, a greater penetration of intermittent renewables requires large-scale flexible energy storage. This need, combined with the growing interest in the use of hydrogen in mobility and industry, makes tangible the prospect of including this energy vector in our daily lives. However, the problems related to the development of dedicated infrastructures make its positioning in the market complex. In a transition phase, power-to-gas systems constitute an emerging solution that allows the use of existing structures for natural gas and, at the same time, solves the problem of hydrogen storage. In this study, a power-to-gas system producing synthetic methane from wind energy was modeled. Management strategies for both the electrolysis system and the hydrogen storage were implemented. In particular, the impact of the storage on the mitigation of the operating condition fluctuation of the methanation unit was analyzed. To verify the influence of the sizing of the subsystems over the system performance, a sensitivity analysis on the sizes of the methanation unit and hydrogen storage was carried out. The conducted sensitivity analysis suggested selecting the smallest size of the hydrogen storage and the size of the methanation unit equal to 80% of the electrolysis system nominal production to achieve good performances of the power-to-gas system. However, the sizing of the subsystems depends on the context in which the power-to-gas system is integrated (i.e., characteristics of wind source), the possibility to valorize the hydrogen surplus (i.e., proximity to hydrogen users), and the specific objective for which the system has been designed
Seawater desalination through reverse osmosis driven by ocean thermal energy conversion plant: Thermodynamic and economic feasibility
No full textSeawater desalination is an effective way to reduce water scarcity affecting many world areas. However, desalination processes require considerable power that might not be available in remote areas, especially in insular zones. Ocean Thermal Energy Conversion (OTEC) is a promising technology to be coupled with reverse osmosis (RO) in remote tropical areas thanks to the constant electricity production, the high number of equivalent hours, and the zero carbon emissions. In this study, the thermodynamic and economic feasibility of a system made of OTEC and RO for freshwater production is assessed to determine the optimal design parameters of the plant. A numerical model was created in Aspen Hysys to simulate all the main OTEC parts, including heat exchangers, pumps, turbine, and seawater pipes. For the RO unit, average performance parameters from the literature were considered. A sensitivity analysis was carried out as a function of evaporation and condensation temperatures of the cycle and riser pipe length. Furthermore, two different fluids, ammonia and R1234yf, and two different materials for the heat exchangers, stainless and polyvinylidene difluoride (PVDF), were compared. The maximum second law efficiency of 26.1% was reached with ammonia at condensation and evaporation temperatures of 13.5( degrees)C and 23( degrees)C, respectively, and a cold seawater depth of 800 m. Despite its higher specific cost, the adoption of stainless steel led to lower water production costs compared to PVDF because of the lower heat transfer areas required. The minimum cost of water of 2.56 $/m3 was obtained with ammonia at a cold seawater depth of 650 m for condensation and evaporation temperatures of 11 C-degrees and 25 C-degrees respectively
Hydrogen from renewables: Is it always green? The Italian scenario
No full textGreen hydrogen production through water electrolysis is recognized as a pillar for the energy transition toward a complete decarbonized economy. The aim of this study is to investigate the potentiality of electrolytic hydrogen (suggesting stricter definitions of green and yellow hydrogen) from grid electricity in Italy and its impact on the reduction of renewable power curtailment and CO2 emissions for the scenarios with increased photovoltaic and wind capacities up to 5 times the 2019 ones. Several electrolysis overall capacities, defined on the basis of the curtailment duration curve, were analyzed. Results showed that hydrogen production based only on the curtailed energy (green hydrogen) leads to a very low utilization factor of the electrolyzers (no more than 22% in the highest penetration scenario) and consequent high specific hydrogen production costs. The annual production of green hydrogen is small in all analyzed cases and far from the stated Italian and European targets. A minimum electrolysis utilization factor should be imposed to reduce the hydrogen production cost by using additional natural gas-generated electricity (yellow hydrogen). However, in this way, specific emissions increase and become even greater than those of the most diffuse steam methane reforming process when the utilization factor imposed doubles the green hydrogen one. The sensitivity analysis showed the influence of the capital cost of the electrolyzers and of the electricity cost for additional non-curtailed energy on the levelized cost of hydrogen
Techno-economic analysis of hydrogen production from PV plants
No full textHydrogen production through electrolysis from renewable sources is expected to play an important role to achieve the reduction targets of carbon dioxide emissions set for the next decades. Electrolysers can use the renewable energy surplus to produce green hydrogen and contribute to making the electrical grid more stable. Hydrogen can be used as medium-long term energy storage, converted into other fuels, or used as feedstock in industry thus contributing to decarbonise hard-to-abate-sectors. However, due to the intermittent and variable nature of solar and wind power, the direct coupling of electrolysers with renewables may lead to high production fluctuations and frequent shutdowns. As a consequence, accelerated electrolyser degradation and safety issues related to low load operation may arise. In this study, simulations of hydrogen production with an electrolyser fed by a PV system are performed in Matlab for a reference year. The effect of PV power fluctuations on the electrolyser operation and production is investigated. The impact of the electrolyser size for a fixed nominal power of the PV plant is also analysed from both energetic and economic points of view
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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